The use of electric brakes in aircraft systems has become increasingly popular. In such systems, electric motors are employed to drive a piston through gears, screws and the like, into a pressure plate of a brake disc stack. Typically, there are a plurality of actuators about each brake assembly, generally uniformly distributed thereabout.
Control of electric braking requires that the actuator clamping force be known and adjusted similar to adjusting the pedal pressure for a hydraulic brake. With hydraulics, a change in pressure is proportional to a change in clamping force and the brake control is based on this. No similar, simple relation exists with electric actuation as the input power measured in voltage and current can vary widely with clamping force depending on whether the motor is moving fast or slow, forward or reversing, or under load. When no practical feedback sensor can be integrated into the electric actuator, force estimation methods are employed often based on motor position from a reference point, the assumed stiffness of the brake frame, motor current draw and other factors. These become complex algorithms and at best provide only an estimated force output that can be in relatively large error due to changes in the frame stiffness, twist in the frame during braking, and uneven brake disk wear, to name a few. Therefore, it is highly desirable to have a reliable and independent sensor of motor clamping force as the current invention describes.
In electric brake systems, it is important that the brake force applied by the actuators be controllable and, in that regard, readily monitored. In the past, load cells have been proposed and used. In such systems, the load cells have been placed upon the brake actuator housing or carrier plate. However, various problems incident to this structure have been encountered. Particularly, as the carrier plate and/or housing deform under load, the load cells themselves deform and lose accuracy. Moreover, deflection and deformation of the carrier plate or housing, and consequent deformation of the load cells, adds a non-axial component to the force reading. However, it is known that the effective braking force from an electric brake disc actuator is axial. Accordingly, the load cell in the prior art has been found to read components that are not brake force components, and which either do not result in the generation of brake force, or result in more brake force than what was requested by the controller. This greatly reduces the accuracy of the data obtained from such load cells, since only the axial component of any actuator's force contributes to brake force. As a consequence, accuracy and integrity of the monitoring system is greatly compromised.